1. Field of the Invention
The present invention relates to a method of making a coated abrasive article comprising a backing having at least two coatings bonded thereon, wherein the abrasive nature of the coatings differs. In one aspect, the abrasive nature can based on the lack of abrasive particles.
2. Discussion of the Art
Abrading or polishing operations sometimes occur where a finer finish is desired on one portion of a surface of a workpiece than on another portion, such as thin film rigid disks for the computer industry. A conventional method of producing such a final surface is to sequentially contact the workpiece with separate abrasive articles having different abrasive natures relative to each other. For instance, the entire workpiece surface may first be finished with a coarse abrasive article, leaving a rough finish, followed by finishing the portion thereof needing a fine finish with a fine abrasive article. An alternate method is to finish the entire workpiece with a fine abrasive article thus imparting a fine finish, and then selectively roughening the surface with a coarse abrasive article to provide the rougher section desired.
For example, thin film disks, commonly used in the computer industry, require different area of the disk to have s a fairly consistent surface texture within each respective area for the disk to perform properly. The texture provided on the surface of a thin film disk is a compromise between the surface finish necessary for the memory area versus that necessary for the head landing zone. The landing zone, a 1/8 inch to 3/8 inch (0.32 to 0.95 cm) wide annular ring at the inner diameter of the disk requires a relatively rough finish to minimize the stiction and friction between the disk and the read/write head on startup and shutdown of the drive. The surface roughness of the landing zone preferably has an Ra of about 40 to 60 angstroms. In contrast, the memory retention area of the disk need not be as rough, but is preferred to be about 20 angstroms Ra. The lower Ra minimizes asperities on the disk surface and enables lower flying heights of the read/write head which results in higher recording densities.
In other applications, a sequence of abrasive grades is used to impart the desired finish on a workpiece. A coarser abrasive article is used first to remove any large amounts of stock, after which a finer abrasive article is used to remove undesirable deep scratches from the coarse abrasive article. This step sequence requires the use of several separate grinders or a grinder that can run several abrasive articles simultaneously. This process requires the operator to move the workpiece to a different machine area, either by moving several steps to a different machine, or moving from one side of a machine to another (if it has the capability to run more than one belt at a time). At times, it may even be necessary to change the belt on the grinder due to equipment constraints, which contributes to a significant loss of productive time. What is desired in the field is to have two or more diverse abrasive natures directly next to each other on the same abrasive article so that effort can be saved on the part of the operator and thus productivity improved.
Art of interest in this area is set forth below.
U.S. Pat. No. 449,930 (Dubey) discloses a sandpaper having multiple sections having various kinds of abrasive particles thereon, the sections being divided by grooves devoid of abrasive particles.
U.S. Pat. No. 875,936 (Landis) discloses an abrading material comprising two different grades of abrasive particles applied to a backing in relatively wide and narrow parallel strips or regions arranged alternately with regions devoid of abrasive provided between the strips.
U.S. Pat. No. 4,930,266 (Calhoun) discloses an abrasive article for ophthalmic lens polishing where the surface of the article has abrasive composites comprising binder and abrasive mineral, arranged in a manner so that the outer edge of the article has a higher density of composites than the center.
JP 4-210383 published Jul. 13, 1992 discloses an abrasive tape for the polishing of magnetic recording medium where the hardness of the binder is varied across the width of the tape to produce different surface finishes.
U.S. Pat. No. 5,152,917 (Pieper et al.) discloses an abrasive article comprising precisely shaped structured abrasive composites.
U.S. Pat. No. 5,167,096 (Eltoukhy et al.) discloses an abrasive pad comprising inner and outer regions of different compressibilities which produce a deeper-groove texture at the inner diameter of a computer disk.
EP 0 554 668 (Calhoun) published Aug. 11, 1993, discloses an abrasive article comprising precisely spaced, oriented abrasive composites which comprise abrasive particles dispersed in a binder. Several grades of abrasive particles can be dispersed in each composite, particularly where one grade is above another.
U.S. Ser. No. 08/514,491 (Strecker) filed Aug. 11, 1995, discloses a method of texturing a thin film rigid disk using an abrasive tape wherein the tape has at least two regions of differing abrasive nature. The two regions can be coated in situ, laminated together on a carrier web, or formed by treating the article such as by calendering or flexing.